Property+Demo.swift

import UIKit

public enum Style {
    case dark
    case light
    case `default`
}

protocol Coolable {
    var isCool: Bool { get set }
}

protocol Stylable {
    var style: Style { get set }
}

extension UIViewController: Stylable, Coolable {
    private func holder<T>() -> Property<T> { Property.init(self) }

    public var style: Style {
        get { holder().wrappedValue ?? Style.default }
        set { holder().wrappedValue = newValue }
    }

    public var isCool: Bool {
        get { holder().wrappedValue ?? false}
        set { holder().wrappedValue = newValue}
    }
}

func test() {
    let a = UIViewController()
    let b = UIViewController()
    a.isCool = true
    b.isCool = false
    print(a.isCool) // true
    print(b.isCool) // false
}